For all but the simplest of samples, NMR spectroscopy yields spectral distributions of considerable complexity. One aspect of this complexity arises from the interaction of nuclear spins among themselves, e.g., coupling of spins to yield a resultant angular momentum. Spin decoupling is the generic method of reducing this complexity of NMR spectra so that the chemical shift of one nuclear species may be observed without spectral effects due to another nuclear species.
A simple example of the decoupling problem and practice is to be found in the heteronuclear molecule. By convention, the (non-zero spin) nuclear constituents I and S are present and it is desired to study the spectral distribution of the I spins without the spectral complications arising from scalar coupling of the S spins to the I spins. Decoupling is achieved if, while observing the I spins, the S spins are repeatedly inverted. Ideally, if this exchange of S spin states is carried out rapidly in comparison with the I-S coupling, the spin-spin coupling will vanish.
In modem approaches to decoupling, the S spins are irradiated with modulated rf pulses in accord with any of several recognized techniques. One class of such pulses are realized as composite pulses, i.e. a sequence of phase shifts over appropriate durations to impose a desired corresponding sequence of rotations of the S spins. Adiabatic sweep pulses are another form of obtaining the desired inversion operation for decoupling the (S) spins. For this purpose, the effective field, Beff is rotated through an angle θ with respect to the x axis over the range π/2≦θ≦−π/2 such that dθ/dt<<γBeff. These pulse decoupling operations are implemented over a separate RF channel, e.g., independent of the observe channel. This pulse decoupling is generically referenced herein as “decoupling radiation” and with careful design the operation may yield effective decoupling over a wide spectral range (broadband decoupling). It is emphasized that the present invention may be practiced with a wide variety of prescriptions for this decoupling radiation and the invention is not to be confused with a particular type of decoupling radiation. Broadband decoupling is generally described by R. Freeman, “Spin Choreography,” chpt. 7, Oxford Univ. Press, Oxford (1997).
Inasmuch as these broadband decoupling techniques comprise cyclic modulated sequences of pulses, artifact in the form of cyclic sidebands will appear in the decoupled spectrum (observe channel) of the instrument at the cycling rate and its harmonics. A number of approaches to suppression of these cycling sidebands have been employed in the prior art. The degree of suppression for these sidebands represents a limit to the examination of true, but weak signals.
One prior art method of sideband suppression applies adiabatic inversion to the (S) spins during the interval between excitation and refocusing of the I spins, and again after refocusing of the I spins through the acquisition of the I spectra. Sideband suppression is achieved coherently by incrementing the delay between decoupling and acquisition. Representative discussion may be found in the references: Ē. Kup{hacek over (c)}e, J. Magn. Reson. 129, 219 (1997); R. Freeman and Ē. Kup{hacek over (c)}e, NMR Biomed. 10, 372 (1997); Ē. Kup{hacek over (c)}e and R. Freeman, J. Magn. Reson. A 115, 273 (1995).
It is known in prior art to suppress decoupling sidebands by decoupling at two different power levels. A high power spin-lock is applied at the beginning of the decoupling sequence. The sidebands are suppressed coherently by increasing the spin-lock time in small increments. See Ē. Kup{hacek over (c)}e, R. Freeman, G. Wider, and K. Wuethrich, J. Magn. Reson. A 122, 81(1996).
It is known to apply decoupling radiation during acquisition of the signal asynchronously throughout the acquisition period. Dispersive components are also removed and spectral distortions are minimized. While a very simple approach, sideband suppression is ordinarily no more than a factor of two by this method. A similar prior art method permutes pulses of the composite pulse decoupling. Although a relatively high degree of sideband suppression is obtained, this is a one dimensional technique, limited to composite pulse decoupling, that randomizes the distribution of the sidebands rather than suppressing them coherently. A. J. Shaka, J. Keeler, and R. Freeman, J. Magn. Reson. 53, 313 (1983). Other methods employ a dispersion of sidebands over a restricted spectral region. A suppression factor of 5-10 may be achieved in favorable situations. See Z. Star{hacek over (c)}uk, Jr., K. Bartu{hacek over (s)}ek, and Z. Star{hacek over (c)}uk, J. Magn. Reson. A 107, 24 (1994); T.-L. Hwang, M. Garwood, A. Tannus, and P. C. M. vanZijl, J. Magn./Reson. A 121,221 (1996); R. W. Dykstra, J. Magn. Reson. 82, 347 (1989).